The effect of suspended fine sediment on protist growth and behaviour was investigated using kaolinite-dominated clay with a mean grain size of 0.9 mum. Suspended sediment particles about the same size as potential food items of picovorous organisms, i.e. bacteria, were expected to interfere with feeding. Growth rates of Halteria sp. and Tetrahymena pyriformis were 0.056 and 0.15 h(-1) irrespective of the clay concentration, Cyclidium sp. showed slightly higher but significant growth rates in the presence of clay, as did most flagellates. The largest difference in growth was found for Spumella sp., which showed rates of 0.19 and 0.14 h(-1) with and without clay, respectively. An exception was the gliding flagellate Entosiphon sulcatum, which was covered by clay particles and therefore hardly came in contact with food particles. Flagellates discriminated strongly against sediment particles prior to food uptake. With decreasing particle concentration this selection mechanism became less important compared to differential digestion especially in Spumella sp. and an unidentified groundwater flagellate. Both Bodo saltans and Monosiga ovata showed differential digestion as well, but their importance in overall selectivity was low even at a low particle concentration. In contrast to flagellates, the ciliates collected many particles, including clay particles and bacteria, and incorporated them into 1 food vacuole. Discrimination against clay was of minor importance in ciliates, but a high feeding capacity seems to compensate for this low selectivity. In contrast to many zooplankton species, it can be concluded that protists are generally well adapted to environmental situations of high sediment load.